Weknowtheplanetusedtohave a muchthickeratmosphere, becausethereusedtobeliquidwateronthesurface, andmaintainingthatrequires a certainamountofatmosphericpressure.
That's a bigengineeringchallenge, becauseMars's atmosphereissothinthatthere's barelyenoughairfor a helicopter's bladestopushagainstandgetitselfairborne.